“Super-slugs” are invading British gardens – and we don’t know how to stop them

Mmmmmm, slimy. Image: Xauxa Håkan Svensson.

The Daily Mail calls it a “slime wave”. The Sun calls them “an army”. Either way, both papers have reported 500bn slugs are set to invade British gardens, after a mild winter created perfect breeding conditions. The Conversation

So is the UK really about to be overwhelmed by slimy slugs? The simple answer is no, but there could be something far worse in store.

Headline numbers alone aren’t necessarily something to get in a lather over. A typical garden can contain several thousand slugs, and the “500bn” figure is derived from estimates of maximum numbers per area. In any case, slug numbers can rise and fall a great deal across time and space, in natural cycles, and even astonishingly dramatic increases are not always cause for concern. Like waves crashing against a beach, the rise is often transient and local – usually slug numbers will drop back to normal, with the disturbance hardly noticed beyond a few local gardeners.

What is more problematic is the progressive, sustained and perhaps less spectacular rise in numbers which, tsunami-like, is maintained for far longer, and spreads widely throughout the countryside. This is Britain’s real slug invasion. So what can we do about it?

The trigger seems innocuous enough in isolation: a few non-native slugs from continental Europe have accidentally been introduced. Several of these species have close relatives in the UK, so similar in fact that only specialists can tell them apart, and they can interbreed freely. Of course, many animals can create hybrids without presenting a threat, but what makes slugs different – and these hybrids so worrying – is their interesting and deviant sex lives.

This is a hybrid between a ‘Spanish stealth slug’ and the UK’s common black slug. Image: Les Noble/author provided.

Slugs are hermaphrodites, which means the same individual exists as both sexes; they first develop as males, before experiencing a true hermaphrodite phase to become female. This means they can dispense with normal mating requirements, and this is where the consequences of the difference between British and continental species becomes significant.

Why British slugs are different

When slugs colonised the UK after the last ice age, they found an island recently covered with ice sheets, where the biological diversity remained poorer than continental Europe. In these circumstances, the ability to self-fertilise was a good evolutionary strategy, one which ensured reproduction even when slug populations were devasated by harsh weather.

A downside of such continued close inbreeding (and mating with oneself is as inbred as it gets) is a rapid loss of genetic variability, and some British slug species eventually came to consist of almost genetically identical individuals. This meant they were more vulnerable to parasites and pathogens that could rapidly evolve to overcome their defences.

Meanwhile, in continental Europe, slugs were becoming more diverse, as balmier weather meant parasites and pathogens were a bigger issue than finding a mate. These slugs tended not to self-fertilise, and were genetically highly variable. This made at least some of them more resilient to attacks from parasites – a possibility not afforded to the inbred British slugs.

Echoes of these different past environments resonate in contemporary species. British slugs, adapted to a variable climate and dearth of mates, have fallen into the clichéd “No sex please, we’re British” mould, producing fewer, bigger eggs later in life by self-fertilisation. Continental slugs, meanwhile, adapted to resist rapidly evolving enemies. Their strategy is therefore to produce many smaller eggs earlier in life, which maximises genetic diversity and compensates for losing many individuals to infection.

The ‘Spanish slug’, one of Britain’s key invaders. Image: tviolet/creative commons.

These different adaptations weren’t an issue until humans disturbed the natural order by moving slugs back and forth as stowaways in commercial produce. As a result of this, we’ve seen widespread breeding between British and continental species. These new hybrid “super-slugs” are highly fertile, and their genetically diverse offspring are adapted to cope with both the British climate and parasites and pathogens, most of which remain in continental Europe anyway.


Fighting the slug invasion?

Legislation aimed at environmental protection has led to the EU banning commercial use of molluscicides (pelleted chemicals which poison slugs but cause collateral damage to other wildlife). Instead, the emphasis is on using natural enemies like nematode worms, though these are generally ineffective against the larger invasive hybrids.

Nonetheless, the increased slug biomass could still host important veterinary or agricultural parasites and pathogens, spreading more plant and animal diseases. Remarkably, despite their obvious presence in our gardens, we remain startlingly ignorant of the fundamental biology of slugs; evidenced by recent work which increased the number of identified British species by more than a fifth.

So where are we going with this phenomenon? Studies have already found invasive slugs and snails can destabilise ecosystems and reduce biodiversity in the US and Scandinavia. Something similar is happening here in the UK.

The good news is that our research suggests population sizes do eventually begin to decline, after 30 to 40 years. The ecosystem may eventually rebound from this slug invasion, but it remains to be seen how long it will take and what the lasting effects will be for the spread of diseases, ecosystem services, or British biodiversity.

Leslie Noble is a reader in zoology at the University of Aberdeen.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

How can we stop city breaks killing our cities?

This couple may look happy, but they’re destroying Barcelona. Image: Getty.

Can’t wait to pack your bags and head off on holiday again? It used to be that people would look forward to a long break in summer – but now tourists have got used to regular short breaks through the year. We love to jet off to the world’s glittering cities, even if only for a day or two. The trouble is, binge travelling may be killing the places we visit.

You may even have seen some “tourists go home” graffiti on your last trip, and it’s not hard to see why. Barcelona is a good example of how a city can groan under the weight of its popularity. It now has the busiest cruise port, and the second fastest growing airport in Europe. Walking through the Barcelona streets at peak season (which now never seems to end) flings you into a relentless stream of tourists. They fill the city’s hot spots in search of “authentic” tapas and sangria, and a bit of culture under the sun. The mayor has echoed residents’ concerns over the impact of tourism; a strategic plan has been put in place.

It is true though, that cities tend to start managing the impact of tourism only when it is already too late. It creeps up on them. Unlike visitors to purpose-built beach destinations and national parks, city-break tourists use the same infrastructure as the locals: existing systems start slowly to stretch at the seams. Business travellers, stag parties and museum visitors will all use existing leisure facilities.

‘Meet the friendly locals’, they said. Image: Sterling Ely/Flickrcreative commons.

Barcelona may only be the 59th largest city in the world, but it is the 12th most popular with international visitors. Compared to London or Paris, it is small, and tourism has spiked sharply since the 1992 Olympics rather than grown steadily as in other European favourites like Rome.

Growth is relentless. The UN World Tourism Organisation (UNWTO) even speaks about tourism as a right for all citizens, and citizens are increasingly exercising that right: from 1bn international travellers today, we will grow to 1.8bn by 2030, according to UNWTO forecasts.

Faced with this gathering storm, just who is tourism supposed to benefit? Travellers, cities, residents or the tourism industry?

Market forces

Managing the impact of tourism starts by changing the way destinations market themselves: once the tourists arrive, it’s too late. Tourism authorities need to understand that they are accountable to the city, not to the tourism industry. When the city of Barcelona commissioned the University of Surrey to look into how it might best promote sustainable development, we found a series of techniques which have been incorporated, at least in part, into the city’s 2020 Tourism Strategy.

In the simplest terms, the trick is to cajole tourists into city breaks which are far less of a burden on the urban infrastructure. In other words, normalising the consumption of sustainable tourism products and services. In Copenhagen, 70 per cent of the hotels are certified as sustainable and the municipal authority demands sustainability from its suppliers.

Higher than the sun. A primal scream from the world’s cities? Image: Josep Tomàs/Flickr/creative commons.

Destinations must also be accountable for the transport impact of their visitors. The marketing department might prefer a Japanese tourist to Barcelona because on average they will spend €40 more than a French tourist – according to unpublished data from the Barcelona Tourist Board – but the carbon footprint we collectively pay for is not taken into account.

Crucially, for the kind of city breaks we might enjoy in Barcelona, most of the carbon footprint from your holiday is from your transport. Short breaks therefore pollute more per night, and so destinations ought to be fighting tooth and nail to get you to stay longer. It seems like a win for tourists too: a few extra days in the Spanish sun, a more relaxing break, and all accompanied by the warm glow of self-satisfaction and a gold star for sustainability.


Destinations can also target customers that behave the most like locals. Japanese first-time visitors to Barcelona will crowd the Sagrada Familia cathedral, while most French tourists are repeat visitors that will spread out to lesser-known parts of the city. Reducing seasonality by emphasising activities that can be done in winter or at less crowded times, and geographically spreading tourism by improving less popular areas and communicating their particular charms can also help reduce pressure on hot spots, much like Amsterdam is doing.

Turnover is vanity, and profit margins are sanity. No city should smugly crow about the sheer volume of visitors through its gates. If tourism is here to stay, then the least cities can do is to sell products that will have the greatest benefit for society. Whether it’s Barcelona, Berlin, Bologna or Bognor, there should be a focus on locally and ethically produced products and services which residents are proud to sell. Tourist boards should work with small businesses that offer creative and original things to do and places to stay, adding breadth to the city’s offering.

The ConversationWhether Barcelona will introduce these ideas will depend on the bravery of politicians and buy-in from the powerful businesses which are happily making short-term profits at the expense of residents and the planet. It is possible to do things differently, and for everyone to benefit more. It may be that the tipping point lies in the age-old mechanics of supply and demand: bear that in mind next time you’re booking a quick city break that looks like it’s only adding to the problem.

Xavier Font is professor of marketing at the University of Surrey.

This article was originally published on The Conversation. Read the original article.